Sensory Organization For Balance; Vision; Vestibular Input - biodex BALANCE SYSTEM SD Operation & Service Manual

CONTENTS

SENSORY ORGaNIZaTION FOR BaLaNCE

Perhaps the most confusing part of a balance evaluation is the part that examines the sensory sys-
tem and its contribution to balance. The sensory system includes the eyes, ears, vestibular appara-
tus (inner ear), somatosensory system (touch and proprioception), taste, and smell. The parts of
the sensory system that contribute directly to balance are the visual, vestibular, and somatosensory
(touch and proprioception) systems. The use of multiple systems in balance allows us to learn new
movements quickly and to fine-tune and easily repeat familiar movements.
The sensory system receives input from the environment through specialized receptors located
in the sensory end-organs in the eyes, vestibular apparatus of the inner ear, muscle spindles,
Golgi tendon organs, and touch receptors in the skin. Sensory input is transmitted to the spinal
cord via afferent nerve fibers and then to the brain via spinal nerve tracts such as the spinothala-
mic tract (pain and temperature) and the dorsal column medial lemniscal tract (fine touch, mus-
cle and tendon position sense).
Sensory input provides a continuous flow of information to the CNS, which in turn utilizes this
incoming information to make decisions about movement. The CNS sifts, compares, weighs,
stores, and processes sensory input and uses this information to alter the force, speed, and range
of a movement.

Vision

Vision is a critical part of our balance system. It allows us to identify objects and determine their
movement and tells us where we are in relation to other objects (object-to-object orientation).
When we use vision to gather information about the position of our body in the environment or
to determine the position of one body part vis à vis another, then vision is providing proprio-
ceptive information to the CNS as well (visual proprioception).
Vision works in conjunction with the vestibular system, comparing information about velocity
and rotation from the vestibular system with actual visual information. The visual system is a
combination of both central and peripheral vision, although some research has suggested that
peripheral vision is more important for postural control and balance than central vision
(Shumway-Cook & Woollacott, 2001).
The visual system may provide inaccurate information to the nervous system. For example, a
person sitting at a stoplight in a car may think she has started to move when the car next to her
starts to move. The visual system "goes along" with the movement of the neighboring car and
tells the brain that both cars are moving. The CNS mediates this sensory conflict by instructing
the leg to slam on the brake to stop the car from moving forward. As soon as the foot touches
the brake the somatosensory and vestibular systems realize that the car is, in fact, not moving.
For a split second, input from the visual system was given preference by the brain, even though
the information turned out to be inaccurate.
Visual input may also be inaccurate due to diseases or disorders that affect the visual system,
such as diabetic retinopathy, cataracts, macular degeneration, injuries, or stroke.

Vestibular Input

The vestibular system is responsible for processing information about movement with respect to
gravity —specifically, rotation, acceleration/deceleration, and head stabilization during gait. The
vestibular system works in conjunction with the visual system to stabilize the eyes and maintain
posture during walking (vestibular-ocular reflex). Vestibular disorders cause a feeling of dizzi-
ness and unsteadiness. Vestibular dysfunction also affects the ability of the CNS to mediate
intersensory conflicts such as that in the example given above.
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